The present invention relates to an X-ray lithographic mask blank or, more particularly, to a reinforced X-ray lithographic mask blank free from warping or distortion and capable of giving a mask for high-precision X-ray lithographic patterning.
Along with the trend in recent years in the manufacture of semiconductor devices toward more and more increased fineness in the pattering on the surface of a substrate such as a semiconductor silicon wafer, the traditional photolithographic patterning method is under continuous replacement with the X-ray lithographic method which is highlighted in respect of the higher resolution in patterning. This X-ray lithographic patterning is conducted by using, like the photomask used in the photolithography, an X-ray lithographic mask which is a frame-supported X-ray permeable membrane having a thickness of 10 .mu.m or smaller and bearing a desired pattern formed from an X-ray impermeable material. It is conventional that the frame supporting the membrane, which usually cannot be thick enough to keep high rigidity, in turn is supported by a reinforcing member adhesively bonded thereto so as to be completely freed from any wrapping or distortion otherwise unavoidable in handling.
The material forming the X-ray permeable membrane is selected from boron nitride, silicon nitride, silicon carbide and the like composed of light elements alone having a small X-ray absorption coefficient and highly resistant against chemicals and high-energy beam irradiation while the X-ray impermeable patterning material is selected from heavy metals or inorganic compounds of a heavy metal such as gold, tungsten, tantalum and the like having a large X-ray absorption coefficient. It is also understood that the X-ray permeable membrane should be in a tensioned condition with an internal stress of 5.times.10.sup.8 to 5.times.10.sup.9 dyn/cm.sup.2 in order to ensure good stability of the membrane form and the X-ray impermeable pattern formed thereon.
The frame as the substrate to support the membrane is usually a mirror-polished silicon wafer having a thickness of 0.3 to 2.0 mm in respect of the susceptibility to etching with an alkaline solution in the process for the preparation of the X-ray lithographic mask blank. Namely, deposition of the membrane material, e.g., silicon carbide, takes place on the surface of the silicon wafer in a suitable thickness to form a deposited film by the sputtering method, chemical vapor-phase deposition (CVD) method and the like and then the silicon wafer is subject to etching from the surface reverse to the deposited film of silicon carbide and the like leaving a circumferential area to serve as a frame supporting the membrane. This is the reason for the limited thickness of the substrate as the frame in consideration of the too long time taken for etching to remove the silicon wafer if it has a large thickness.
High-purity silicon per se is a relatively brittle material with low mechanical strengths so that the silicon wafer as the substrate frame to support the membrane is subject to warping or distortion in preparation and handling of the lithographic mask if it is not broken. Accordingly, it is sometimes necessary that the frame supporting the membrane is provided with a reinforcing member adhesively bonded thereto. In view of the requirement for an approximately equal thermal expansion coefficient with the silicon wafer forming the frame, several grades of borosilicate glass are used as the material of the reinforcing member. It is usual that such a glass-made reinforcing member is bonded to the frame by using an organic adhesive such as an epoxy adhesive, having a high adhesive bonding strength.
When the frame and the reinforcing member are bonded together by using an adhesive, the thickness of the adhesive layer cannot be small enough and is usually in the range from 30 to 100 .mu.m in order to ensure a high adhesive bonding strength. Such a large thickness of the adhesive layer causes a serious problem that even a relatively small unevenness in the thickness of the adhesive layer may adversely affect the parallelism between the reinforcing member as the base of the mask and the frame or, hence, the frame-supported membrane, while the parallelism should desirably be perfect within a deviation of a few micrometers. Moreover, even the very small difference in the thermal expansion coefficients between a silicon wafer and borosilicate glass, the values being 2.4.times.10.sup.-6 /.degree.C. and 3.5.times.10.sup.-6 /.degree.C., respectively, cannot be neglected when the requirement for the accuracy of patterning is extremely high because distortion of the frame may take place even by a relatively small change in temperature.
The above mentioned problem due to the difference in the thermal expansion coefficients between the materials of the frame and the reinforcing member can of coarse be solved when they are made from one and the same material. Namely, it is proposed in Japanese Patent Kokai No. 2-162714 to form the reinforcing member from a single crystal of semiconductor silicon which is the same material as that of the membrane-supporting frame. A problem in this case consists in the method for adhesively bonding the frame and the reinforcing member in order to obtain a practical adhesive bonding strength. As proposed in the above patent document, the frame and the reinforcing member are directly contacted with each other and the assembly is heated in a non-oxidizing atmosphere at a temperature of 900.degree. C. or higher or, preferably, 1000.degree. C. or higher for a length of time of at least 30 minutes or, preferably, at least 60 minutes to cause seizing therebetween. The above mentioned conditions for the heat treatment may be beyond the limit of the thermal stability of the X-ray permeable membrane so that a change is unavoidably caused in the internal stress of the membrane to affect the accuracy of the pattern thereon even if it eventually be not broken.
It would be an alternative idea that, prior to the deposition of the film of silicon carbide and the like on one of the surfaces of the silicon wafer, a frame-formed reinforcing member made from a silicon single crystal having a sufficient thickness is bonded to the other surface of the silicon wafer be the above mentioned method of high-temperature direct bonding followed by the deposition of the silicon carbide film and processing thereof into a frame-supported membrane. This method, however, cannot provide a complete solution of the problem because the heat treatment at such a high temperature sometimes causes warping or distortion of the bonded body and decrease in the smoothness of the surface of the silicon wafer on which the film of silicon carbide and the like is to be subsequently deposited if not to mention the cost for the installation of a high-temperature furnace.
Japanese Patent Publication 39-17869 teaches that two silicon wafer can be bonded together when they are directly contacted and heated in an oxidizing atmosphere under a higher pressure such as oxygen at about 650.degree. C. for 1 as a result of formation of a thin layer of silicon oxide on the contacting surfaces.